#include <bits/stdc++.h>
using namespace std;
int main() {
    long long a, b; cin >> a >> b;
    long long d = a + b;
    for (long long c = 2; c * c <= d; c++) {
        if (d % c == 0) {
            if ((a + c) % b == 0 && (b + c) % a == 0) {
                cout << c << endl;
                return 0;
            } 
        }
    }
    cout << -1 << endl;
    return 0;
}